Bird Mating and Parental Behaviors Linked to Population Demographics

Variations in bird mating behaviors, parental care, and sexual ornamentation are primarily driven by population demographics. This finding comes from an international team of researchers from the UK, China, Germany, and Hungary. They studied 261 bird species across 69 avian families.

The research indicates that a skewed adult sex ratio (ASR) in bird populations is caused by demographic factors. While male and female chicks are typically born in equal numbers, their survival rates as juveniles and adults often differ. This can lead to one sex being more likely to die before reproducing, creating an imbalanced sex ratio.

Previous research suggested that sex ratio influences mate selection, promiscuity, and parental cooperation. The new study clarifies that demographics are the cause, not the consequence, of these behaviors. For example, parental duties or breeding behaviors do not cause sex differences in survival that reinforce a sex ratio imbalance.

When one sex is more common, sexual selection intensifies, leading to greater competition for mates. In male-biased species, females tend to be larger, more ornamented, or more competitive. Conversely, in female-biased species, males are often larger and more ornamented. The great bustard, for instance, is a female-biased species where males are significantly larger than females.

This research supports a unidirectional evolutionary pathway. Demography shapes the sex ratio, which then influences social behavior. If one sex consistently experiences higher mortality, perhaps due to competition, infection, or predation, the resulting sex ratio bias can affect sexual selection, sex roles, and mating behaviors across generations. These findings were published in the journal *Nature Communications*.